This invention relates to a method, reagent and kit for the colorimetric determination of nitrate ions in aqueous solutions.
The quantitative determination of nitrate ions is becoming of increasing importance in analysis. Nitrate content in drinking water and service water, in foodstuffs, in agricultural chemistry and in general environmental analysis is increasingly becoming a general criterion of quality. Reliable and economic determination procedures are required for monitoring the amounts of nitrate present in drinking water and the fertilizer residues in foodstuffs, and for determination of the level of nitrate in soils which are used for agriculture, permitting cost-effective mineral fertilizing.
There are relatively few methods for the determination of nitrate ions. The only procedures of importance are of the colorimetric type, and even these are less satisfactory than those for other ions.
There are essentially two different procedures for the determination of nitrate ions, direct nitration and/or oxidation in concentrated sulfuric acid and reduction, with suitable reducing agents, of the nitrate ions to nitrite ions and their determination as an azo dye after the so-called Griess reaction.
Oxidation reactions with anthranilic acid (deep-blue solution), brucine (orange product) or diphenylamine (blue product) have the advantage of a marked reaction contrast, but have the disadvantage of low selectivity, since other ions, for example chloride in sea water, likewise have oxidizing effects under these conditions. Nitration reactions of phenols lead to yellow nitrophenols. In this case, the higher selectivity, even though still hardly adequate for practical purposes, is counteracted by the yellow reaction colors obtained, the differences in intensity of which can be differentiated by eye only with difficulty.
A method for the determination of nitrates using bis-phenols or bis-phenol ethers is described in U.S. Pat. No. 4,424,277.
A method for the determination of nitrates using sodium salicylate is described, in Lange/Vejdelek, Photometrische Analyse (Photometric Analysis), Verlag Chemie Weinheim, 7th Edition, 1980, page 365, with the yellow 5-nitrosalicylic acid obtained being determined by photometry. Using that method, it is necessary to remove interfering cations with an ion exchanger, since, for example, nitrates intensify and bromides and iodides reduce the intensity of the color obtained. The determination is very elaborate; the sample containing the reagent must be evaporated to dryness by heating at 100.degree.-120.degree. C. After cooling, 96% sulfuric acid is added to the dried residue. Then, only after the addition of water, alkali solution, making up to the mark in a graduated flask with water and standing for 20 minutes, can the determination be carried out.
In general, the previously known methods are lengthy, require a considerable expenditure in apparatus, and are more or less susceptible to interference or are too sensitive for practical requirements.